Slot fed stub antenna

ABSTRACT

A slot fed stub antenna comprising a ground plane and a cable geometrically centered and normally aligned with said ground plane, having one end in abutment with said ground plane, wherein said cable comprises an elongated hollow member, dielectrically supported, having a circumferentially slotted cross section and a wire axially centered within said hollow member.

United States Patent 91 Rosenbluth SLOT FED STUB ANTENNA [75] Inventor: Murray Rosenbluth,

Hiawatha, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22] Filed: Dec. 13, 1971 211 Appl. No.: 207,295

Lake

[52] US. Cl. ..343/769, 343/705, 343/861 [51] Int. Cl. ..HOlq 13/12 [58] Field of Search ..343/769, 771, 829,

[56] References Cited UNITED STATES PATENTS Buchwalter et al ..343/769 Brown ..343/830 May 22, 1973 1/1953 Nelson et a] ..343/830 8/1954 Van Atta ..343/873 Primary ExaminerEli Lieberman Attorney-Harry M. Saragovitz [57 ABSTRACT A slot fed stub antenna comprising a ground plane and a cable geometrically centered and normally aligned with said ground plane, having one end in abutment with said ground plane, wherein said cable comprises an elongated hollow member, dielectrically supported, having a circumferentially slotted cross section and a wire axially centered within said hollow member.

3 Claims, 5 Drawing Figures PAIENIED HAYZZIQTS SHEET 1 [)F 2 SLOT FED STUB ANTENNA The invention described herein may be manufactured, used, sold and licensed by or for The Government of the United States for governmental purposes without payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION The present invention is directed to transceiver (i.e., transmitter and receiver) antennas. More specifically, it is concerned with those stub type antennas that are fed by a circumferential slot located on the stub. This type of antenna is particularly useful in applications wherein space limitations exist.

The instant invention permits the integration of the antenna stub, matching section, and a protective cover (a radome) into a single package which can be located in a nose or tail section, outside of a shell or missile body.

As a result of certain current improvements in large artillery shells and missiles, particular on-board space problems have developed. These problems have led to the development of a class of integrated antennas that provide a variety of field patterns while occupying only negligible space within the vehicle. Included in this class of antennas is the stripline antenna discussed in the following references: (1) The Radiation Conductance of a Series Slot in a Strip Transmission Line, by A. A. Oliner, IRE Convention Record, Part 8, Mar. 1954, pp. 89-90, and (2) Evaluation of Airborne Telemetry Antennas for use in UHF Telemetry Bands NWC-TP 5099 by V. R. Christenson, Naval Weapons Center, China Lake, Cal., 1971. Also, included in the class of integrated antennas is the TEM line antenna described in The Slotted TEM Line Antenna, IEEE Transactions on Antennas and Propagation, pgs. 261-3, Mar., 1968.

The current trend in the development of antennas has been to search for new techniques which yield space conservation within the missile or shell. Accordingly, the cavity backed slot (See Slot Antennas, 'by N. E. Lindenbladt, IRE Vol. 35, No. 12 (1947)) has acquired the form of a stripline slot (Oliner ref. above) and has been integrated with the matching elements and power dividers of the stripline form (Christenson ref.). These integrated antennas are placed on the missile or shell exterior.

The TEM line antenna referenced above provides a unique solution to space conservation problems on board a missile. Its matching element can be either external or internal to the missile or shell. Power dividers are not required. This antenna is also placed externally to the missile skin or shell body.

The present'invention, called the slot-fed-stub, belongs to the above class of integrated antennas. These antennas, if properly designed, have the potential to re sist large shocks and very high temperatures. Also, they are useful at very high altitudes.

The most pertinent prior art in the field known to this writer is disclosed in FIG. 1 which illustrates the antenna disclosed in the TEM line article. This antenna comprises a series of tubular segments disposed on a ground plane 12, with an axially disposed conductor 14 threaded through said tubular segments.

When in operation, the prior art antenna produces an asymmetrical highly directional electromagnetic (EM) field pattern. This asymmetry is due to the location of the segments 10 with respect to the ground plane 12.

This location renders the production of any field but an assymetrical EM field pattern impossible.

Due to this lack of EM field pattern symmetry the prior art antenna is limited to applications in which there is an absence of spin. Were such an antenna to be used in a spin environment, the assymetry of the EM field pattern would preclude the antenna from functioning properly during a significant segment of each rotation of the missile or shell, thus limiting signal reception during a portion of the spin. Hence, it is evident that the existent state of the art has expressed a need for an antenna that is both compact and onmidirectional. More particularly, requirements for such an antenna have long existed in the electronics technology relating to military projectiles and missiles.

SUMMARY OF THE INVENTION An object of the present invention is to provide an integrated, compact and omnidirectional microwave antenna for use in shells and missiles.

A further object is to provide a microwave antenna that will eliminate the present need for bulky matching networks located internally to the missile.

Another object is the perfection of an antenna which can resist high temperatures, very large impacts, persistent vibrations and high altitudes.

A yet further object is to provide an antenna having radiation characteristics similar to a stub antenna, however, having a slot located on, and feeding, the stub.

Still another object is to provide a matching technique for use with the slot-fed-stub.

The present invention comprises a ground plane and a cable geometrically centered and normally aligned with said ground plane, having one end in abutment with said ground plane, wherein said cable comprises an elongated hollow member, dielectrically supported, having a circumferentially slotted cross section and a wire axially centered within said hollow member.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the prior art.

FIG. 2 is a side view of the present invention with the radome removed.

FIG. 3 is a top view of the invention shown in FIG. 2 taken along line 3-3.

FIG. 4 is a perspective view of the invention partly in section along line 4-4 of FIG. 3.

FIG. 5 is a diagrammatical circuit representation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION FIGS. 2, 3 and 4 illustrate an embodiment of the slotfed-stub. A transceiver 16 is placed in abutting relationship to an antenna 17. The antenna comprises a (preferably circular) ground plane 18 and a (preferably semi-rigid cable 20 that is axially and normally aligned with the ground plane 18. The cable comprises an elongated hollow member (a stub) 21, comprising first and second segments 21A and 21B axially separated by a circumferential slot or gap 24, and a wire 22 that is axially centered within said hollow member 21 and electrically insulated therefrom. One end 27 of the hollow segment 21A abuts the ground plane 18 and is mechanically and electrically coupled thereto. The hollow portion of said member 21 is filled with a dielectric material 25 that envelops the wire 22. In order to give both structural support and protection to the antenna 17, a

covering radome 26 is utilized. Such a radome is particularly significant in artillery and missilery applications. The radome may be constructed as a spherically parabolic enclosure having the interior of its polar region in physical contact with the end of said cable 20.

The slot 24 need not necessarily be parallel to the ground plane 18; however, in such a condition the EM field symmetry may become distorted. It should also be noted that the slot 24 can be replaced by a plurality of slots, although this is generally not necessary.

The cable 20 is preferably linear, since any nonlinearity would complicate the matching procedure which will be described below.

The wire 22 is of lesser length than the outer hollow member 21. An optional feature that may help facilitate compact construction of the antenna is a disc 28 which is placed at the end of the hollow segment 218. The effect of such a disc is to cause the stub-fed-slot to appear electrically longer in terms of wavelengths.

Prior art matching techniques at microwave frequencies have involved the variation of two paramaters to obtain a matched condition. These techniques have included the following: (a) Variation of the line length from the load, and insertion of a matching stub length. This technique is known as single stub matching. (b) Variation of line lengths from the load, and then use of an appropriate transformer. This approach is called transformer matching. And, ((2) Variation of the stub lengths in a double stub tuner. This is called double stub matching.

Through variation of the length of 1) that segment 21A of the hollow member 21 located between the ground plane 18 and the slot 24, and (2) the length of the wire 22, the reactance of the total system can be cancelled out, thus obtaining a resonant condition. Also, through said procedure the resistance of the antenna and transceiver can be equalized, thus matching the total system. The matching of resistances is conventionally sought at 50 ohms.

The technique used for matching the stub-fed-slot also comprises the variation of two parameters. These are denoted as Lm and Lr on FIG. 2. Other parameters of importance in the present matching technique are L and Ls (both shown. on FIG. 2). The sequence of steps in the present matching method comprises: (A) setting Lo equal to between 0.15 and 0.20 of a free space wavelength; (B) setting Lm equal to L0; (C) setting Ls equal to between 0.005 and 0.010 of a free space wavelength; (D) adjusting Lr to a length which yields a system input resistance ofless than 1 ohm (normalized to 50 ohms); (E) decreasing Lm until the impedance of the antenna has a resistive value of greater than 1 ohm; (F) decreasing Lr until the reactive component of the input impedance is cancelled out; and (G) repeating steps (E) and (F) until an input impedance of 1 ohm with zero reactance is approximated.

The theory underlying the above method may be understood by reference to FIG. 5 which is a diagrammatical circuit representation of the slot-fed-stub antenna. Xm represents the reactance of the matching section Lm. Ra represents the resistance of the slot-fed-stub. Za represents the impedance of the unmatched slotfed-stub antenna. 20 is generally inductive in nature, as represented by La. Zin represents the total input impedance of the slot-fed-stub antenna.

By reduction of the length Lm, the capacitative reactance Xm is increased to a point at which the inductive component of 2a is ultimately cancelled out, thus placing the slot-fed-stub in a condition of resonance. By reducing the length Lr, the resistive component of Za is varied until the resistance of the antenna and transceiver are equalized thus effecting a matching of the total system.

The above matching process is facilitated through the use of an impedance analyzer (such as a Hewlett Packard 8410 displaying a Smith Chart.)

It is thus seen from the above discussion that the objects set forth in the Summary of the Invention are among those made apparent from, and efficiently attained by, the antenna and method of the preceding description.

I wish it to be understood that I do not desire to be limited to the exact detail of construction shown and described for obvious modification will occur to persons skilled in the art.

Having described my invention, what I claim as new, useful and non-obvious, and accordingly, by this instrument, secure by Letters Patent of the United States, is:

1. A slot-fed-stub antenna comprising:

a ground plane; and

a cable geometrically centered and normally aligned with said ground plane which comprises:

a first elongated hollow segment having one end in abutment with said ground plane and mechanically and electrically connected thereto;

a second elongated hollow segment axially aligned with said first elongated hollow segment and separated therefrom by a circumferential slot;

a dielectric material filling said first and second hollow segments; and

a wire coaxially positioned within said dielectric material and said first and second hollow segments, wherein said wire measured from said ground plane is shorter in length then the length of said hollow member and electrically insulated therefrom.

2. In the structure described in claim 1, a method of impedance matching in which the antenna to be matched has the following parameters: a cable length L0 running from (a) that edge of the circumferential slot furthest from the ground plane to (b) the free end of the cable; a length Lm of the axially centered wire, said length running from (a) that edge of said slot furthest from the ground plane to (b) the end of said wire; a width Ls, said width being the width of the slot; and a cable length Lr running from the ground plane to the edge of said slot closest to the ground plane, wherein said method comprises the steps of:

A. setting L0 equal to between 0.15 and 0.20 ofa free space wavelength;

B. setting Lm equal to L0;

C. setting Ls equal to between 0.005 and 0.10 of a free space wavelength.

D. adjusting Lr to a length which yields a system input resistance of less than one ohm (normalized to 50 ohms);

E. decreasing Lm until the input impedance of the antenna has a resistive value of greater than 1 ohm; and

F. decreasing Lr until the reactive component of the input impedance is cancelled out.

3. The method as recited in claim 2 in which said method further comprises repeating steps (E) and (F) until an input impedance of 1 ohm with zero reactance is approximated. 

1. A slot-fed-stub antenna comprising: a ground plane; and a cable geometrically centered and normally aligned with said ground plane which comprises: a first elongated hollow segment having one end in abutment with said ground plane and mechanically and electrically connected thereto; a second elongated hollow segment axially aligned with said first elongated hollow segment and separated therefrom by a circumferential slot; a dielectric material filling said first and second hollow segments; and a wire coaxially positioned within said dielectric material and said first and second hollow segments, wherein said wire measured from said ground plane is shorter in length then the length of said hollow member and electrically insulated therefrom.
 2. In the structure described in claim 1, a method of impedance matching in which the antenna to be matched has the following parameters: a cable length Lo running from (a) that edge of the circumferential slot furthest from the ground plane to (b) the free end of the cable; a length Lm of the axially centered wire, said length running from (a) that edge of said slot furthest from the ground plane to (b) the end of said wire; a width Ls, said width being the width of the slot; and a cable length Lr running from the ground plane to the edge of said slot closest to the ground plane, wherein said method comprises the steps of: A. setting Lo equal to between 0.15 and 0.20 of a free space wavelength; B. setting Lm equal to Lo; C. setting Ls equal to between 0.005 and 0.10 of a free space wavelength. D. adjusting Lr to a length which yields a system input resistance of less than one ohm (normalized to 50 ohms); E. decreasing Lm until the input impedance of the antenna has a resistive value of greater than 1 ohm; and F. decreasing Lr until the reactive component of the input impedance is cancelled out.
 3. The method as recited in claim 2 in which said method further comprises repeating steps (E) and (F) until an input impedance of 1 ohm with zero reactance is approximated. 